In T-1 digital carrier systems, a channel bank samples voice signals using pulse amplitude modulation. The channel bank subsequently converts the pulse amplitude modulated (PAM) signal to an eight bit encoded digital signal, interleaves the encoded signal with 23 other voice channels and transmits an output bit stream at 1.544 megabits per second over a line.
According to well known sampling theories, an analog waveform such as a voice signal can be digitally represented if samples are taken at least every 125 microseconds. Thus, the cyclic basis of sampling, multiplexing and de-multiplexing in many telecommunication switching systems uses a basic system clock rate of 8 kHz. Accordingly, voice channels in a channel bank are typically sampled 8,000 times per second or slightly more than twice the upper frequency limit of voice signals passable by most telecommunication switching systems (i.e., 4 kHz).
Channel banks typically comprise a number of line cards having voice and/or data ports for interfacing subscriber equipment with a digital network. Some line cards can include means for tone generation to simplify calibration and/or troubleshooting, or to indicate call processing states, for example. These line cards interface with a PAM bus. The PAM bus connects to a converter unit that converts PAM signals from the subscriber lines into pulse code modulated (PCM) voice signals for processing by the digital network.
Tone generation in a number of existing PAM bus interfaces such as channel bank line cards is subject to distortion, that is, the signals are conventionally generated using analog means, then processed two times. The analog-to-PAM conversion process generates some distortion to which more distortion is added during PAM-to-digital conversion. Thus, the quality of the sine wave created using the sampled signal degrades.
A need exists for a circuit which generates a 1 kHz tone having a sampling rate of 8,000 samples per second or eight samples per cycle at a low cost.